The present invention relates in general to improved metallization systems for semiconductors or devices and integrated circuits, and in particular to contacts, such as Ohmic contacts or Schottky barrier contacts, for high temperature semiconductor devices and to methods and compositions for use in the manufacture of such metallization systems.
There are many important applications where it is desirable to use electrical transducers, discrete semiconductor devices or complete circuits capable of sustained operation at elevated temperatures; for example, at temperatures up to 500.degree. C. and above. Such applications include in situ monitoring of jet engines, internal combustion engines, turbines, nuclear reactors, geothermal holes, and "hot" oil wells. The high temperatures and often corrosive environments encountered in these applications, however, present extremely difficult reliability and lifetime problems which are not being met with existing semiconductor devices or technologies. Particularly, severe are the problems of hermetic packaging; internal device passivation; and metallurgical proglems associated with the formation of Ohmic contacts to the semiconductor, metal interconnects, and bonded lead wires.
A crucial step in developing reliable high temperature devices of the type described is believed to be that of isolating a semiconductor which is capable of functioning at elevated temperatures, and a metallization system that is compatible with the semiconductor during high temperature operation. Compatibility requires that: (1) the chosen metal or metals exhibit good adhesion to the semiconductor, (2) acceptable microstructural and mechanical properties are exhibited at the operating temperatures, (3) the selected metal/semiconductor system is capable of producing good Ohmic (non-blocking or non-injecting) characteristics with minimal specific contact resistance at the operating temperatures, and (4) the overall system is stable in its electrical, structural and mechanical properties for periods of at least several thousand hours at the operating temperatures.
In the design and fabrication of semiconductor devices and integrated circuits, metals are used to perform at least four separate functions; namely, to form (1) Ohmic contacts to the semiconductor; (2) rectifying or Schottky contacts to the semiconductor; (3) interconnects; and (4) gate metallizations in MOS devices.
THe physical properties demanded of the metal are slightly different in each case, so that no one elemental metal or alloy is optimum for all four functions. The search for appropriate metals for functions (1) and (2) is largely empirical, because there is no way to predict, a priori, whether a given metal will form good quality Ohmic or Schottky contacts with a given semiconductor. Function (3) requires that the metal have a high electrical conductivity to minimize I.sup.2 R losses and parasitic resistances. With respect to function (4), high conductivity is less important than the metal's work function (which plays a role in determining the MOS threshold voltage). In addition to these considerations, the choice of an appropriate metallization should take into account practical constraints imposed by processing and reliability considerations.
Such practical constraints include at least the following processing considerations: (a) compatibility with practical deposition methods; (b) compatibility with standard patterning techniques; (c) compatability with standard wire bonding techniques; (d) good adhesion to the semiconductor and passivating layers; and (e) compatability with thermal cycling. The applicable reliability considerations include: (f) good edge definition and line-width control; (g) good thermal conductivity; (h) resistance to electromigration; (i) resistance to interdiffusion or reaction with substrates; and (j) resistance to formation of intermetallic compounds.
Those constraints, combined with the basic functional requirements, place severe restrictions on the choice of metallization. In practice, some of the constraints require conflicting properties; e.g., reactivity vs. inertness, making it difficult to find a single, simple metallization which will suffice for all functions. This often leads to the use of multicomponent or multilayer metallizations with various different metals serving as adhesion layers, diffusion barriers, bonding or capping layers, etc.
Moreover, during processing, the metal layers are often exposed to high temperatures and, during operation, they may be exposed to moderately elevated temperatures, temperature gradients and high current densities. These environmental conditions serve to accelerate diffusion and chemical reactions among most metal, semiconductor, and insulating materials leading, in many cases, to the formation of unwanted compounds which can cause device failure for electrical or mechanical reasons.
From the consideration of the required semiconductor characteristics, such as intrinsic carrier concentration, it is clear that compounds such as GaAs and GaP are among the most attractive candidates for high-temperature applications above 200.degree. C. While present technology can be used to produce satisfactory Ohmic contacts to GaAs and GaP, these methods are not capable of being applied to high temperature devices.
Most metals, when deposited onto a substrate by sputtering, evaporation, or plating, produce films having a polycrystalline microstructure. This is true of all metals and alloys currently used in semiconductor devices and integrated circuit metallizations. During pattern delineation, the grain boundaries often etch preferentially, leading to poor edge definition and poor line-width control. It has been found that the best edge resolution is obtained with fine-grained metals. As the grain size is reduced, however, grain-boundary diffusion becomes increasingly troublesome and all processes controlled by diffusive transport, such as phase separation, compound formation, etc., are enhanced. Electromigration, a very severe problem occurring at high current density (J.gtorsim.10.sup.4 A/cm.sup.2), is also enhanced along grain boundaries. As a result, in conventional metallizations, those films which afford the highest pattern resolution pose the most severe reliability problems due to grain-boundary diffusion and electromigration.
It is believed that simple metallization systems using the minimum number of component overlayers have the best chance for success in achieving high-temperature stability over long durations. A prime requirement for high stability of a contact overlayer is resistance to interdiffusion reactions, which implies an overlayer component that is effectively a diffusion barrier. To satisfy this requirement, the overlayer material that is used should have a high kinetic resistance to any change or modification of its atomic structural state at the working temperature of the device.
This invention takes a novel and unique approach to this problem by making use of the characteristic of some metal and alloy systems to undergo a vitrification to a compositionally homogeneous amorphous phase. The as-deposited amorphous films have good adhesion and show at least an order of magnitude improvement in corrosion protection compared to polycrystalline coatings. Moreover, annealing treatments below the glass transition temperature have been found to enhance further this protective behavior of amorphous films.